Solar cells are components that convert light into electric energy. Normally, they are made of semiconductor materials comprising regions or layers having a different conductivity for positive and negative charge carriers, n-type or p-type conductive regions. The regions are referred to as emitters and absorbers. Positive and negative excess charge carriers created by incident light are separated at the p-n junction between the emitter layer and the absorber layer, and they can be collected and dissipated by contact systems that are electrically conductively connected to the appertaining regions. Accordingly, the only excess charge carriers that contribute to the useable electric output of solar cells are those that reach the contact systems and do not recombine with a charge carrier of the opposite polarity prior to that.
Single-sided contact solar cells have both contact systems in order to separately collect the excess charge carriers from the absorber layer on one and the same side. First of all, this has the fundamental advantage that only one side has to be processed for contact purposes. As set forth in the present invention, the term “front contact” is used when both contact systems are located on the side (front) of the solar cell that is exposed to incident light during operation. In contrast, the term “back contact” is used when both contact systems are located on the side (back) of the solar cell that is not exposed to incident light during operation. Moreover, the term “top” is used in conjunction with the solar cell. This refers to the side of the solar cell that is accessible during operation and especially also during its production. In the case of an absorber wafer, both sides of the solar cell are accessible and thus referred to as tops. In the case of thin-layer-based solar cells with a substrate or superstrate, the side of the solar cell opposite from the substrate or superstrate is referred to as the “top”. With a substrate, this is the front, whereas with a superstrate, this is the back.
An important aspect in the arrangement of the contact systems is primarily their efficiency during the collection of charge carriers. If the absorber layer of the solar cell is of sufficiently good electronic quality, that is to say, if the effective bulk-diffusion length of the minority charge carriers is substantially larger than the thickness of the absorber layer, then as a rule, the emitter layer can advantageously be located on the back of the solar cell. In the case of a back contact, this especially translates into the advantages that, first of all, no shading losses occur through a contact system, which leads to an improvement in the efficiency of the solar cell, and secondly, the side of the solar cell that is to be exposed to the incident light during operation can be simply covered with additional functional layers over its entire surface. This can be, for example, a front field passivation layer (Front Surface Field, FSF) for backscattering the minority charge carriers or else an additional anti-reflection layer. However, if the absorber layer is of relatively low electronic quality, that is to say, if the effective bulk-diffusion length of the minority charge carriers is smaller than or in the order of magnitude of the thickness of the absorber layer, then the emitter layer should advantageously be located on the front of the solar cell. All of the minority charge carriers of the absorber layer that are created at a depth that is less than the effective bulk-diffusion length of the absorber layer can then be reliably collected. In the case of front contact, for purposes of improving the efficiency of the backscattering of the minority charge carriers, a back field passivation layer (Back Surface Field, BSF) can be provided (analogously, in the case of a back contact, a front field passivation layer (Front Surface Field, FSF) can be provided).
A relevant aspect for solar cells according to the invention is single-sided back contacts according to the state of the art that also contact the front by means of plated-through holes through the appropriately structured absorber layer. These are so-called Metal-Wrap-Through (MWT) or Emitter-Wrap-Through (EWT) technologies in which a metallic rib that contacts the front emitter layer via a contact grid or the front emitter layer itself are plated-through through the absorber layer with a corresponding contact system on the back of the solar cell.
Single-sided front contact solar cells with plated-through holes have not yet been realized due to a lack of a technologically simple and efficient production method. Only one-sided back-contact solar cells with plated-through holes are known from the prior art. A good overview of back-contact solar cells with plated-through holes can be found in the publication by E. V. Kerschaver et al.: “Back-contact Solar Cells: A Review” (Prog. Photovolt: Res. Appl., May 25, 2005, published online in Wileys InterScience DOI: 10.1002/pip.657).
An emitter-wrap-through (EWT) technology for wafer-based solar cells is described in U.S. Pat. No. 5,468,652. This publication describes point-contacting in which holes that are laser-drilled through the emitter layer, which is located on the front of the absorber layer, and through the absorber layer are contacted with a contact system on the back of the wafer. The emitter layer—as well as an optional BSF layer—are created by means of diffusion. Here, the other contact system for dissipating the minority charge carriers is insulated with respect to the back of the wafer and interdigitated with a contact system for dissipating the minority charge carriers that is not insulated with respect to the back of the wafer. In particular, a structuring separation of the emitter layer and absorber layer or BSF layer on the back, including the two interdigitated contact systems, is needed. This is done by selective removal of an insulating oxide layer and by selective diffusion.
An alternative emitter-wrap-through technology for wafer-based solar cells is known as a RISE-EWT solar cell (see publication by P. Engelhardt et al.: “The RISE-EWT Solar Cell—A New Approach Towards Simple—High Efficiency Silicon Solar Cells”, 15th International Photovoltaic Science and Engineering Conference, Shanghai, China, 2005, p. 802-803). The structuring separation of the emitter layer and absorber layer or BSF layer on the back, including the two interdigitated contact systems on the back of the wafer, is carried out here by means of laser structuring (creation of comb-like depressions) so that a metal can be deposited over the entire back surface, thereby forming the two contact systems.
Furthermore, DE 696 31 815 T2 describes a wafer-based back-contact crystalline homo-solar cell without plated-through holes in which a contact grid surrounded by an insulation layer is provided above which a contact layer is arranged over the entire surface, with an interposed insulation layer. With this homo-contact solar cell, however, the emitter layer is structured by counter-doping the absorber layer with dopants from the contact grid. Therefore, the emitter layer is not configured as an autonomous functional layer but rather is made up of integrated small regions in the absorber layer directly underneath the contact grid. Owing to the complex diffusion processes, a sharp p-n junction cannot be made. The insulation layer on the metal grid can be formed by a selective oxide, for example, aluminum oxide.
DE 198 54 269 A1 describes a thin-layer-based hetero-solar cell with a substrate and with plated-through holes in which one contact system is configured as a contact grid, but it is integrated directly into the back of the absorber layer in front of an electrically conductive substrate. The other contact system is configured as a full-surface contact layer and is arranged on the back of the electrically conductive substrate (FIG. 6). Consequently, the electrical conductivity of the substrate is essential for the function. The contact grid between the absorber layer and the substrate is completely sheathed by an insulation layer in order to avoid a direct and indirect electrically conductive connection of the absorber layer to the contact grid. The electrically conductive connection of the contact grid exclusively to the emitter layer is the result of plated-through holes through the absorber layer in the form of passage openings through the full-surface emitter layers and absorber layers. The passage openings are partially filled with emitter material and partially with contact grid material, which is why they are difficult to realize technologically. FIG. 3 shows an embodiment with two interdigitating contact systems that—embedded in an insulation layer—are applied directly onto the substrate. For contacting purposes, two different point contacts are needed. Towards this end, holes are made through the emitter layer, the absorber layer and the insulation layer that, in the case of point-contacting of the emitter layer have to be created in two stages and have to be lined with emitter material; in the case of point-contacting of the absorber layer, only the lower region of the holes is selectively filled with metal. As an alternative, the contact system that is provided for contacting the absorber layer can be deposited onto the substrate without insulation. It is then contacted directly at the time of the deposition of the absorber layer. This saves the step of point-contacting of the absorber layer, but instead, the contact layers on the substrate have to be appropriately structured so as to be insulating/non-insulating. However, in all of the embodiments, both contact systems are located on the bottom of the solar cell (the side closest to the substrate) underneath the active solar cell layers, as a result of which they are commensurately difficult to produce and to contact.
A thin-layer-based superstrate solar cell in an n++ip-doped configuration having a back contact with two contacting types for the n++-layer and the p-layer is described in WO 03/019674 A1. The intrinsically doped i-layer can be considered here as the absorber layer and the n++-layer or p-doped layer can be considered as the emitter layer or BSF layer. Here, both contact systems are on the top of the solar cell (the side furthest away from the superstrate, which is, at the same time, the back of the solar cell) above the active solar cell layers (consisting of an emitter layer, an absorber layer and a field passivation layer), and these contact systems are plated-through by point contacts through an insulation layer to the p-layer, or through the insulation layer, the p-layer and the i-layer to the n++-layer. In the case of the p-layer, the point contacts are metallic, and in the case of the n++-layer, the holes have to be lined with an emitter material (an n-layer). The different point contacts are now combined by means of contact strips on the top of the solar cell to form the two structured contact systems. In particular, the possibility also exists of an integrated series connection and parallel connection of a finished solar cell module.